1. Field of the Invention
The invention relates to a process and apparatus for making video discs, more particularly according to a technique for moulding a flexible substrate on a master carrying information in the form of micro-reliefs.
2. Description of the Prior Art
Optically readable video discs are generally such that the information is recorded along a spiral track or a track in the form of concentric circles, formed by a succession of elements whose dimensions are on the order of a micron and micro-channels or projections according to the process which may more generally be called micro-reliefs are obtained for example by thermal ablation.
For some applications, it may be useful to have at one's disposal a more or less large number of identical copies, obtained from the same original disc: either a specially engraved master which will only serve for duplication or a discs capable of being recorded and read by the user. Numerous copying processes are already known, particularly those using pressing techniques such as those used for audio discs, which require the use for the copies of a thermo-deformable plastic material. The pressing processes, well adapted to mass production, require heavy, cumbersome and expensive apparatus, which is scarcely advantageous for small-scale production and cannot be used by the user himself.
More adapted to small-scale production are the moulding processes which include contacting a master and a substrate, with at least one intermediate moulding agent. The apparatus is then lighter, simpler to use and lower in cost price, able to be installed in any location, such as a duplicating apparatus, for example in the very place of use. Furthermore, the moulding does not generally generate mechanical stresses at the level of the information and provides better fidelity of reproduction than pressing. One of the problems met with is that of air bubbles which insert themselves between the master and the substrate. This problem may be avoided by operating in a vacuum, as is described in French patent application No. 77 22565 filed on July 22, 1977 and published under the number 2 397 931. However, the need to have a vacuum enclosure mechanism results in the fact that the apparatus remains complicated and delicate to use. Another process for moulding flexible disks is described in French patent application No. 74 34629 filed on Oct. 15, 1974 and published under the No. 2 247 329. According to one of the processes described in this patent application there is gradually applied by means of a roller a substrate coated with a curable resin on a master whose information bearing surface has been previously treated. The substrate, previously partially rolled up on the roller, is gradually and evenly applied on the master when the roller rolls over the master thus driving in front of it possible air or gas bubbles. In one of the embodiments provided, the resin may be applied on the master and be formed for example from monomers capable of being polymerized by exposure to ultra-violet light.